Ac voltage control apparatus

ABSTRACT

Alternating current voltage control apparatus comprises a coarse and a fine voltage regulator feeding the primary winding of a buck/boost transformer the secondary of which is connected in series with a load. The coarse voltage regulator has two movable contacts capable of passing one another, and has the ends of the winding connected to the two poles of an alternating current supply of the same frequency as that connected to the load. The fine regulator is connected between one contact on the coarse regulator and one pole of the supply and itself carries a movable contact. The primary of the buck/boost transformer is connected between the other movable contact on the coarse voltage regulator and the movable contact on the fine regulator.

United States Patent [72] Inventor Robert Stewart Paulden Stockport,England [21] Appl. No. 6,696 [22] Filed Jan. 29, 1970 [45] Patented Oct.19, 1971 [73] Assignee Ferranti, Limited Hollinwood, England [32]Priority Feb. 12, 1969 [33] Great Britain [31 7523/69 [54] AC VOLTAGECONTROL APPARATUS 4 Claims, 2 Drawing Figs. [52] U.S. Cl. 323/6, 323/435R, 323/45, 323/47 [51 Int. Cl GOSt 3/04 [50] Field ofSearch 323/435, 44,45, 47, 57, 83, 91, 6 56] References Cited UNITED STATES PATENTS 687,14711/1901 Fleming 323/45 Primary Examiner-J. D. Miller AssistantExaminer-A. D. Pellinen AnorneyCameron, Kerkam & Sutton ABSTRACT:Alternating current voltage control apparatus comprises a coarse and aline voltage regulator feeding the primary winding of a buck/boosttransformer the secondary of which is connected in series with a load.The coarse voltage regulator has two movable contacts capable of passingone another, and has the ends of the winding connected to the two polesof an alternating current supply of the same frequency as that connectedto the load. The fine regulator is connected between one contact on thecoarse regulator and one pole of the supply and itselfcarries a movablecontact. The primary of the buck/boost transformer is connected betweenthe other movable contact on the coarse voltage regulator and themovable contact on the fine regulator.

AC VOLTAGE CONTROL APPARATUS This invention relates to voltage controlapparatus for use with alternating current systems.

Voltage control apparatus for use with alternating current exists inmany forms, a common one being the use of a bucking-and-boosting(buck/boost) transformer having one winding connected in series with theAC supply to the load. The other winding of the buck/boost.transformeris fed with an AC voltage which may be varied betweenrequired limits, for example, by means of a tapped or continuouslyvariable autotransformer If the autotransformer is provided with twomovable contacts to which the two ends of the buck/boost transformerwinding are connected, then it is possible to use the apparatus toeither increase or decrease the load voltage as required.

The main disadvantage with this relatively simple arrangement is that atlow-voltage settings. the voltage changes by relatively large steps, andthe accuracy of control :is lost. The eventual limitation is the finitenumber of turns on the autotransformer over which the moving contactsoperate.

it is an object of the invention to provide voltage control apparatusfor an AC system which will provide small and accurate voltage changesat the low-voltage end of the operating range.

According to the present invention there is provided voltage controlapparatus for use with alternating current systems which includes afirst moving contact autotransformer voltage regulator capable of havingthe ends of its winding connected directly across an alternating currentsupply and carrying two movable contacts capable of passing one anotheralong the regulator winding, a second moving contact autotransformervoltage regulator arranged to. have one end of its winding connecteddirectly to one side of said alternating current supply and havinganother point on its winding connected to one movable contact on thefirst voltage regulator and carrying a movable contact capable oftraversing part at least of the regulator winding, and a bucking andboosting transformer having its primary winding connected between theother moving contact on the first voltage regulator and the movingcontact on the second voltage regulator and with its secondary windingsuitable for connection in series with the load to be across analternating current supply.

The invention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a circuit diagram of one form of the apparatus; and

FIG. 2 illustrates the application of the invention to a three phaserectifier system.

Referring now to FIG. 1, a load is connected in series with thesecondary winding 11 of a buck/boost transformer 12 across an AC supply.A first or coarse'l moving contact autotransformer voltage regulator hasis winding connected across the AC supply which may be the same as thatconnected to the load, and which must in any event be in phase with theload supply. The coarse regulator comprises a winding 13 connected tothe supply, and two movable contacts 14 and 15. Each of the movablecontacts is capable of traversing the entire winding 13 of theregulator. Connected between one of the movable contacts 14 and one sideof the regulator supply the winding 16 of a second or me" regulator ofthe same type. This has a single movable contact 17 arranged to cover avoltage range which is only a fraction of the voltage of the windingl6,say one-third. The primary winding 18 of the buck/boost transformer isconnected between the other movable contact on the coarse regulator andthe movable contact 17 on the fine regulator. The movable contacts maymove over tappings connected to the winding or over the turns of theactual winding.

Consider first the case when the contact 17 on the fine regulator is atthe upper end of winding 16. In this condition contact 17 iselectrically connected directly to contact 14 on the coarse regulator,and the voltage regulator operates in the same manner as the known typeof regulator described above. Whenever the two contacts 14 and 15 occupythe same position on winding 13 then the voltage is applied to theprimary winding 18 of the buck/boost transformer 12 will be zero. Thisis not affected by the actual position of the contacts on winding 13.Whenever the two contacts move away from one another a voltage isapplied to the primary winding 18 of the buck/boost transfonner. Thisvoltage has a phase dependent upon the relative positions of the twocontacts.

For example, if contact 14 moves above" contact 15 as shown in FIG. 1,then a voltage of one phase is applied to the winding 18. If however thecontact 14 moves below the contact 15, then the voltage applied to thewinding 18 is of opposite phase. Hence it is possible to provide,through the buck/boost transformer a voltage which may either be addedto the voltage supplied to the load, or be subtracted from the voltagesupplied to the load.

Such a simple arrangement is, as has been stated, already known, if oneignores the presence of the fine regulator.

The fine regulator operates to add to the coarse regulator output asmall variable fraction of the voltage tapped off the coarse regulatorby the contact 14. The position of contact 14 therefore determines therange of voltage covered by the contact l7, and the position of contact14 may be adjusted to give the required accuracy of control. Whenworking at low voltages the contact 14 will be towards the bottom of thewinding 13, and hence travel of contact l7 on winding 16 covers the]small voltage tapped off by contact 14.

Suppose, for example, that the load is supplied with a voltage which isonly 50 percent of its maximum working voltage. The voltage regulatormay be arranged to increase or decrease this by the same amount, thusproviding full control from zero to percent of the working voltage. Theregulators and buck/boost transformer are connected so that maximumvoltage is applied to load when contact 15 is at the bottom end ofwinding 13 and contact 14 is at the top end of the winding. Contact 17on the fine regulator then provides for variation of the load voltagefrom 100 percent with contact 17 at the top end of its travel toapproximately 83 percent with contact 17 at the lower end of its travel.The lower value arises since the regulator adds to the 50 percentvoltage from the load supply two-thirds of the same voltage. When thevoltage applied to the load is to be a minimum, then the positions ofcontacts 14 and 15 are reversed. If contact 14 is placed at the verybottom end of the winding 13, then the fine regulator exerts no control,but as soon as a small voltage appears across the fine regulator, thenthis may be varied in very small steps by movement of contact 17.

Thus the complete system operates so that the range of control coveredby the fine regulator is always a fixed fraction of the variable outputvoltage, not of the fixed supply voltage, and the accuracy of control ismaintained down to a very low output voltage level.

Although the voltage control apparatus described may be used to vary avoltage over a wide range, it is more commonly used as a voltagestabilizing arrangement which may be motordriven under the control of asensing relay. It is then possible to control the coarse regulatorcontacts so that the fine regulator contact always works about thecenter of its range, controlled by the sensing relay.

FIG. 2 illustrates one possible application for the voltage controlapparatus described above. This shows a three-phase rectifier systemwith the three rectifiers fed from a star-connected transformersecondary winding. The three-phase primary winding is not shown. Intoeach connection from the transformer to a rectifier is inserted thesecondary winding of a buck/booster transformer. Each of the threevoltage regulator arrangements is identical to that shown in FlG. l. Thethree AC supplies to the regulators are of different phase, eachregulator supply being in-phase with the rectifier to which it isconnected via the regulators. Hence the regulator supplies are readilyderived from tertiary windings on the rectifier transformer.

The corresponding movable contacts of the regulators in the threeseparate phases must be kept in step with one another in order tobalance the loads on the three phases of the supply. The variouscontacts will probably be interconnected mechanically in order toachieve this.

it is possible to modify the connections of the fine regulator byconnecn'ng the movable contact 14 to the center of the upper part ofwinding 16, instead of to the top end of the winding. This will thenallow the movable contact 17 to tap off a voltage variable above andbelow the voltage applied to the winding by contact 14.

I claim:

1. Voltage control apparatus for use with alternating current systems,including a first moving-contact autotransformer voltage regulatorincluding a winding, the ends of said winding adapted to be connecteddirectly across an alternating current supply said regulator furtherhaving two movable contacts adapted to pass one another along theregulator winding, a second moving-contact autotransformer voltageregulator including a winding, said second regulator arranged to haveone end of its winding directly connected to one side of saidalternating current supply and having another point on its windingconnected to one movable contact on the first voltage regulator andcarrying a movable contact capable of traversing part at least of theregulator winding, and a bucking and boosting transformer having itsprimary winding connected between the other movable contact on the firstvoltage regulator and the movable contact on the second voltageregulator and with its secondary winding arranged for connection inseries with the load to be controlled across an alternating currentsupply.

2. Voltage control apparatus as claimed in claim 1 in which the movablecontact carried by the second voltage regulator is adapted to traverseonly that part of the regulator winding adjacent to the point on thewinding connectedto the said one movable contact of the first voltageregulator.

3. Voltage control apparatus for controlling the voltage supplied to aload adapted to be connected across an alternating current supplycomprising a first coarse voltage regulator, said coarse voltageregulator including a winding and two movable contacts arranged forpassing one another along the winding, means for connecting said coarsevoltage regulator across an alternating current supply, a second finevoltage regulator, said fine voltage regulator including a winding and amovable contact, means for connecting said winding between one side ofsaid alternating current supply and said movable contact on the firstvoltage regulator, said movable contact on said second voltage regulatorbeing arranged to traverse part at least of the associated winding, abucking and boosting transformer having a primary and a secondarywinding, means connecting said primary winding between the other movingcontact on the first voltage regulator and the moving contact on thesecond voltage regulator and means for connecting said secondary windingin series with the load to be controlled.

4. Voltage control apparatus as claimed in claim 3 in which the movablecontact carried by the second voltage regulator is adapted to traverseonly that part of the associated regulator winding adjacent to the pointon the winding connected to the said one movable contact of the firstvoltage regulator.

1. Voltage control apparatus for use with alternating current systems,including a first moving-contact autotransformer voltage regulatorincluding a winding, the ends of said winding adapted to be connecteddirectly across an alternating current supply said regulator furtherhaving two movable contacts adapted to pass one another along theregulator winding, a second movingcontact autotransformer voltageregulator including a winding, said second regulator arranged to haveone end of its winding directly connected to one side of saidalternating current supply and having another point on its windingconnected to one movable contact on the first voltage regulator andcarrying a movable contact capable of traversing part at least of theregulator winding, and a bucking and boosting transformer having itsprimary winding connected between the other movable contact on the firstvoltage regulator and the movable contact on the second voltageregulator and with its secondary winding arranged for connection inseries with the load to be controlled across an alternating currentsupply.
 2. Voltage control apparatus as claimed in claim 1 in which themovable contact carried by the second voltage regulator is adapted totraverse only that part of the regulator winding adjacent to the pointon the winding connected to the said one movable contact of the firstvoltage regulator.
 3. Voltage control apparatus for controlling thevoltage supplied to a load adapted to be connected across an alternatingcurrent supply comprising a first coarse voltage regulator, said coarsevoltage regulator including a winding and two movable contacts arrangedfor passing one another along the winding, means for connecting saidcoarse voltage regulator across an alternating curRent supply, a secondfine voltage regulator, said fine voltage regulator including a windingand a movable contact, means for connecting said winding between oneside of said alternating current supply and said movable contact on thefirst voltage regulator, said movable contact on said second voltageregulator being arranged to traverse part at least of the associatedwinding, a bucking and boosting transformer having a primary and asecondary winding, means connecting said primary winding between theother moving contact on the first voltage regulator and the movingcontact on the second voltage regulator and means for connecting saidsecondary winding in series with the load to be controlled.
 4. Voltagecontrol apparatus as claimed in claim 3 in which the movable contactcarried by the second voltage regulator is adapted to traverse only thatpart of the associated regulator winding adjacent to the point on thewinding connected to the said one movable contact of the first voltageregulator.